This is an optional Avid RC 2.6x6x3mm "Ceramic" Kyosho/Yokomo Differential Thrust Bearing. This thrust bearing makes working on your diff much easier. You no longer have to worry about losing a diff ball or getting covered in grease when you rebuild your diff! Ceramic thrust bearings are smooth and incredibly hard, and deliver the smoothest diff of any material. However, they have a very narrow setting window and more care must be taken when tightening the diff. These are recommended for experienced racers.